Second Harmonic Generation Crystal Research Articles

Femtosecond-laser-inscribed cladding waveguides in KTiOPO4 crystal for second-harmonic generation and Y-branch splitters

In this study, the fabrication of straight- and Y-branch-cladding waveguides in KTiOPO4 crystals using femtosecond laser-direct writing is described. The second-harmonic generation (SHG) of green light through the cladding waveguides was realized using a pulsed-wave pump at 1,064 nm. The guiding properties of straight-cladding waveguides fabricated by varying the laser-writing conditions were investigated. The minimum insertion loss was approximately 2.0 dB at 1,064 nm. Confocal micro-Raman spectroscopy was used to investigate the lattice micro-modifications. The maximum SHG conversion efficiency of the straight-cladding waveguide reached 31.4% with a maximum output power of 79.29 mW from an input power of 252.6 mW. The performances of the Y-branch splitters with splitting angles ranging from 0.5° to 2.6° were characterized at 1,064 nm, showing excellent properties, including symmetrical output ends and approximately equal splitting ratios. An SHG conversion efficiency of 13.4% and maximum output power of 33.63 mW were achieved in the Y-branch splitter with a splitting angle of 1.0°. These findings support potential applications in constructing compact-frequency converters by using femtosecond laser-written KTiOPO4 depressed-cladding waveguides.

Characterization of organic crystals for second-harmonic generation.

Second-harmonic generation (SHG) is a common technique with many applications. Common inorganic single-crystalline materials used to produce SHG light are effective using short IR/visible wavelengths but generally do not perform well at longer, technologically relevant IR wavelengths such as 1300, 1550, and 2000 nm. Efficient SHG materials possess many of the same key material properties as terahertz (THz) generators, and certain single-crystalline organic THz generation materials have been reported to perform at longer IR wavelengths. Consequently, this work focuses on characterizing three efficient organic THz generators for SHG, namely, DAST (trans-4-[4-(dimethylamino)-N-methylstilbazolium] p-tosylate), DSTMS (4-N,N-dimethylamino-4'-N'-methylstilbazolium 2,4,6-trimethylbenzenesulfonate), and the recently discovered generator PNPA ((E)-4-((4-nitrobenzylidene)amino)-N-phenylaniline). All three of these crystals outperform the beta-barium borate (BBO), an inorganic material commonly used for SHG, using IR pump wavelengths (1200-2000 nm).

67 mJ, 137 ns narrow bandwidth 355 nm UV laser.

A high-efficiency, high-energy, narrow bandwidth, hundred-nanosecond pulse width 355 nm ultraviolet (UV) laser was realized. A high-energy single-frequency 1064 nm fundamental laser was demonstrated firstly with multistage end-pumped preamplifiers and side-pumped main amplifiers. The corresponding pulse energy, repetition rate, pulse duration, bandwidth, and beam quality factor M2 were determined to be 221 mJ, 100 Hz, 156 ns, 2.25 MHz, and 1.23, respectively. By using type-I phase-matching LBO crystal for second harmonic generation (SHG) and type-II phase-matching LBO crystal for the sum frequency generation of the third harmonic, 67 mJ, a narrow bandwidth 355 nm UV laser was obtained with a pulse width of 137 ns and an energy stability of RMS < 1.2%@2 h. The fundamental to UV optical conversion efficiency was 30.3%. Our results provided a new way for generating high-energy, narrow bandwidth hundred-nanosecond 355 nm UV lasers used for direct-detection Doppler wind lidar (DWL) system.

978 Nm All-Polarization-Maintaining Mode-Locked Fiber Laser Based on Phase-Biased Nonlinear Amplifying Loop Mirror

Terahertz (THz) wave generation based on ultrafast laser are one of the most predominant and popular technologies. In particular, for THz generation by photoconductor antenna and optical rectification, higher optical to THz conversion efficiency usually requires ultrafast laser excitation into semiconductor or nonlinear crystal, therefore it is of great desire to develop ultrafast laser with high performance. CW Yb-doped lasers at 980 nm have been applied to generate THz wave range from 0.5 to 6 THz by DFG technique. To meet the demand of some THz wave generation (e.g. BNA crystal, gold nanoplasma), 980 nm ultrafast fiber laser are ideal pump sources and in desire to be developed. Here we demonstrate a 978 nm robust all-polarization-maintaining (PM) picosecond fiber laser mode- locked by phase-biased nonlinear amplifying loop mirror (PB- NALM). By introducing a nonreciprocal - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pi$</tex-math></inline-formula> /2 phase shifter into the fiber loop, a self-starting stable mode-locking is obtained with the center wavelength of 978.14 nm and a 3-dB bandwidth of 0.43 nm. The pulse duration is 8.1 ps corresponding to the time bandwidth product (TBP) of 1.09. The maximum average output power is 14.5 mW with a repetition rate of 45.438 MHz. After a master oscillator power amplification system, the 978 nm average output power reaches 2.12 W, which is then injected into a PPLN crystal for second harmonic generation and 52 mW 489 nm ultrashort lasers is obtained. This is, to the best of our knowledge, the first report of PB-NALM-based mode-locked fiber laser at 978 nm. The 978 nm ultrafast fiber laser and SHG to 489 nm have great potential in terahertz emission by shorter NIR and blue light.

Versatile, compact chirped pulse amplifier pump system for ultrafast optical parametric amplifiers.

We report on the development of a pump system for ultrafast optical parametric amplifiers (uOPA) as an upgrade for the existing uOPA at the Petawatt High Energy Laser for heavy Ion eXperiments (PHELIX) and the new Petawatt ENergy-Efficient Laser for Optical Plasma Experiments (PEnELOPE). The system consists of a two-stage chirped pulse amplifier, centered around a high energy Yb:YAG regenerative amplifier that delivers 108 mJ uncompressed output energy, resulting in 92 mJ at 1030 nm after compression, pulse durations of 1.4 ps, a high beam quality of Mx/y2 = 1.02 / 1.16 and a relative energy stability of 0.35 %. A second harmonic generation (SHG) efficiency of up to 70 % is achievable and a maximum pulse energy of 43 mJ at 515 nm has been obtained, which is only limited by the damage threshold of the SHG crystal. A self-phase modulation stage makes this system a widely applicable, self-seedable pump module for uOPA without placing strong requirements on its seed oscillator.

Growth, spectral and quantum chemical investigation on hexamethylenetetramine 4-nitrophenol monohydrate single crystals for second harmonic generation and optical limiting applications

To grow single crystals of hexamethylenetetramine 4-nitrophenol monohydrate (HMTNP), the solvent evaporation method was used. The single crystal X-ray diffraction (SCXRD) method is used to confirm the crystal lattice parameters and space group. Powder X-ray diffraction, 1H, and 13C NMR, FTIR, and FT-Raman spectroscopy were employed to investigate the title crystal's crystalline nature, molecular arrangement, and presence of functional groups. The Hirshfeld surface study is aimed at investigating the intermolecular charge-transfer interactions that occur in HMTNP owing to the presence of OH···N, CH···O, and OH···O hydrogen bonds. UV-Visible and photoluminescence studies were used to determine the lower cut-off wavelength and emission peaks. Additionally, the title crystal's optical band gap energy has been estimated to be 3.17 nm. Dielectric and photoconductivity investigations of the single crystal revealed dielectric constant values and a negative photoconductivity response. The Second Harmonic Generation (SHG) efficiency of the powder crystal is 4.31 times that of a conventional KDP crystal. In Z-scan and optical limiting experiments, the self-defocusing nature and optical limiting behavior of HMTNP were demonstrated. Furthermore, DFT calculations at the B3LYP/6–311++G(d,p) level of theory were utilized to investigate the formed crystal's molecular structural geometry, molecular vibration, intermolecular charge transfer interactions and hyperpolarizability.

High-efficiency broadband second-harmonic-generation of YCa4O(BO3)3 crystal

High-efficiency broadband second-harmonic-generation (SHG) of YCa4O(BO3)3 (YCOB) crystal was demonstrated for the first time. The theoretical calculation predicted that the retracing point wavelength for type-I SHG of YCOB crystal was at around 1650 nm, which was proved by the followed phase-matching experiment. With a tunable femtosecond laser as the light source, the type-I SHG experiments of YCOB crystal were performed in the range of 1550–1700 nm. Compared with other broadband SHG crystals, YCOB crystal presented the highest optical conversion efficiency of 58%, the largest spectral bandwidth conversion ratio of 65%, and the broadest wavelength acceptance bandwidth of 118.8 nm·cm1/2.

Experimental focusing in the characterization of ultrashort chirped pulses in a simplified device

Grating eliminated no-nonsense observation of ultrafast incident laser light e-fields is an experimental technique for the full characterization of ultrashort laser pulses. It requires the nonlinear conversion of broadband ultrashort optical pulses into their second harmonic generation (SHG) that is relatively tightly focused on a thick SHG crystal. In this paper, the laser focusing problem is experimentally presented and demonstrated in the measurements of 780 nm chirped pulses emitted from a Ti:sapphire oscillator. In this experimental study, it has been shown how the focusing effect can cause distortions in the trace of the chirped pulses. It is demonstrated that the measurement of broadband pulses requires that the beam have a wide angular divergence. Therefore, a tight focusing is essential. A long focal length leads to a smaller range of angles incident on the crystal causing an insufficient divergence angle. Finally, the ability of this device in the characterization of double and complicated chirped pulses is described. The broadening of complex chirped pulses after passing through a thick BK7 glass is measured and the group velocity dispersion of the BK7 glass is calculated as about 47.3 fs2 mm−1. Also the train of double chirped pulses created using a Michelson interferometer is measured and the spectral fringes marked by phase jumps are observed. The experimental results of the retrieved phases and intensities are in good agreement with independently measured data.

Efficient near-infrared laser emission and nonlinear optical properties of a newly developed Yb:LYSB laser crystal

Herein, we report on the growth and characterization of La0.78Y0.32Yb0.04Sc2.86(BO3)4 (Yb:LYSB) crystal as bifunctional laser and nonlinear optical (NLO) medium. High quality Yb:LYSB crystal with non-congruent melting has been grown by the Czochralski technique. Structural, linear and NLO properties, and near-infrared (NIR) laser emission performances of the grown crystal are presented. The absorption cross-section at 980 nm for σ-polarization was evaluated to be 0.6 × 10−20 cm2, and emission cross-section around 1080 nm in π-polarization and σ-polarization amounted to 3.6 × 10−21 cm2 and 1.33 × 10−21 cm2, respectively. The spectroscopic investigations revealed the existence of two different Yb3+ centers in the LYSB crystal matrix. Efficient laser emission at 1028 nm was obtained with a c-cut 4 at.% Yb:LYSB uncoated sample with a thickness of 3.5 mm. Employing the pump at 971.5 nm with a fiber-coupled diode laser, the Yb:LYSB medium yielded laser pulses with 1.5 mJ energy at 4.25 mJ energy of the absorbed pump pulse, corresponding to an overall optical-to-optical efficiency of 0.35. The laser operated with a high slope efficiency of 0.57. The refractive indexes were measured and the NLO properties of Yb:LYSB crystal for second harmonic generation of 1028 nm laser radiation were determined. The results of this work indicate Yb:LYSB as a promising crystal to design new laser sources in the NIR and green (∼514 nm) spectral ranges based on direct laser emission and self-frequency doubling processes, respectively.

BiGa(SeO3)3: A Phase Matchable SHG Material Achieved by Cation Substitution.

The search of novel second-harmonic generation crystals in gallium selenite systems results in three new compounds of MGa2(SeO3)4 (M = Sr, Pb) and BiGa(SeO3)3. The isostructural MGa2(SeO3)4 (M = Sr, Pb) are centrosymmetric (P1̅, no. 2). The structures represent a complicated three-dimensional skeleton containing two different types of Ga4Se4 8-member polyhedral ring tunnels with the counter cations located at the broad ones. When the divalent counter cations were substituted by a trivalent lone pair cation of Bi(III), a novel noncentrosymmetric compound of BiGa(SeO3)3 was achieved. BiGa(SeO3)3 displays a 3D gallium selenite framework with 1D bismuth oxide chains filling in the 12-member polyhedral ring tunnels. Frequency doubling measurements showed that BiGa(SeO3)3 presented an apparent SHG signal comparable to the commercial KDP and can realize phase matching. The lone pair anionic groups of SeO32- made the major contribution according to the SHG density analyses. BiGa(SeO3)3 exhibits a wide transparent range and large optical bandgap (3.90 eV). The laser-induced damage threshold of BiGa(SeO3)3 was estimated to be 98.68 MW/cm2, about 44 × AGS.

Diamond-blade diced trapezoidal ridge waveguides in YCOB crystal for second harmonic generation

Trapezoidal ridge waveguides have been fabricated in YCOB nonlinear optical crystals by carbon ion irradiation and precise diamond-blade dicing. The diced ridges with smooth side-walls allow for near-infrared (1064 nm) light guiding with propagation losses around 1 dB/cm. Refractive index profile of a waveguide has been reconstructed in a reasonable manner. Green second harmonic light have been generated at room temperature via type I birefringent phase matching. Under the pump of continuous and pulsed lasers, conversion efficiencies for guided-wave frequency doubling can be up to ~1.10% W−1 and ~6.22%, respectively.

Efficient high-power continuous-wave lasers at green-lime-yellow wavelengths by using a Nd:YVO4 self-Raman crystal.

Efficient high-power continuous-wave Nd:YVO4 visible lasers at versatile wavelengths of 532 (green), 559 (lime), and 588 nm (yellow) are demonstrated to be achieved by using the identical cavity mirrors and gain medium. A dichroic coating is deposited on one end surface of the gain medium to gather the backward green-yellow emission. The green, lime, and yellow outputs are individually optimized by using different phase-matched lithium triborate (LBO) crystals for second harmonic generation (SHG) of the fundamental field, sum frequency generation (SFG) of the fundamental and the stimulated Raman fields, and SHG of the stimulated Raman field, respectively. At a pump power of 31.6 W, the output powers at 532, 559, and 588 nm can be up to 6.8, 5.4, and 3.1 W. The high efficient and compact Nd:YVO4 lasers at green-lime-yellow wavelengths can be potentially beneficial to future applications in retinal photocoagulation.

Efficient third harmonic generation of 355 nm picosecond laser pulse

Picosecond laser with high-repetition-rate and high pulse energy is widely favorite in many scientific and industrial applications. Some nonlinear crystals can be used to efficiently convert a near-infrared laser into a green laser or an ultraviolet laser which has a higher photon energy and a smaller focal area. Especially for high-quality and high-speed transparent hard material fabrication, green or ultraviolet picosecond laser has been found to possess unique advantages. In this paper, the experiments on high-efficiency second-harmonic-generation (SHG) and third-harmonic-generation (THG) by using a home-made all-solid-state picosecond laser amplifier and an LBO crystal are reported. The picosecond laser amplifier consists of a seed source, a regenerative amplifier and a two-stage single-pass amplifier. The seed source is a commercial all-solid-state picosecond oscillator with a pulse duration of 8.3 ps and a repetition rate of 68 MHz. The repetition rate is reduced from 68 MHz to 500 kHz by an electro-optic Pockels cell (PC), and the period doubling bifurcation is minimized by reducing the duration of high voltage in PC. Both the regenerative amplifier and the two-stage single-pass amplifier are pumped by three 30-W continuous-wave fiber-coupled laser diodes. After the regenerative amplifier, the seed laser is amplified to 4.86 W with a repetition rate of 500 kHz at 1064 nm. Then the laser power is increased to 23.2 W by a two-stage single-pass amplifier, and the M2 value of the amplified laser in the X direction and in the Y direction are 1.330 and 1.235, respectively. The final pulse duration is 13.4 ps, which is slightly stretched in the amplification chain compared with the seed pulse duration (8.3 ps). For high-efficiency SHG and THG from near-infrared to green and ultraviolet, we carefully study the optical characteristics of some nonlinear crystals, such as LBO, BBO, BIBO, CLBO, etc., and we find that the LBO crystal, which has a high damage threshold, small walk-off and high nonlinear coefficient, is the best choice for both SHG and THG. Then the parameters of the two crystals for SHG and THG are specially designed according to the phase matching condition, the walk-off and the laser parameter. As a result, a 4-mm-long type-I phase matching LBO with cutting angle of &lt;i&gt;θ&lt;/i&gt; = 90° and &lt;i&gt;φ&lt;/i&gt; = 11.6° is used for SHG, and a 3-mm-long type-II phase matching LBO with cutting angle of &lt;i&gt;θ&lt;/i&gt; = 42.2° and &lt;i&gt;φ&lt;/i&gt; = 90° is used for THG. Finally, we realize high-efficiency frequency conversion with SHG power of 12.7 W at 532 nm and THG power of 9.25 W at 355 nm. The corresponding optical-optical conversion efficiencies reach 54.7% and 39.6%, respectively.

Inline self-diffraction dispersion-scan of over octave-spanning pulses in the single-cycle regime.

We present an implementation of dispersion-scan based on self-diffraction (SD d-scan) and apply it to the measurement of over octave-spanning sub-4-fs pulses. The results are compared with second-harmonic generation (SHG) d-scan. The efficiency of the SD process is derived theoretically and compared with the spectral response retrieved by the d-scan algorithm. The new SD d-scan has a robust inline setup and enables measuring pulses with over-octave spectra, single-cycle durations, and wavelength ranges beyond those of SHG crystals, such as the ultraviolet and the deep-ultraviolet.

Direct Modulation Capabilities of Micro-Integrated Laser Sources in the Yellow–Green Spectral Range

In this letter, the millisecond modulation capabilities of high power multisection diode laser systems and their application to second harmonic generation (SHG) are investigated. Different modulation methods of the optical output power of a distributed Bragg-reflector tapered diode laser and a master oscillator power amplifier (MOPA), both emitting at 1120 nm, are compared. The modulated optical output power features a rectangular pulse shape with pulse lengths in the low millisecond regime and peak optical power above 5 W. Based on these results, the most suitable method for the modulation of the second harmonic emission is identified and applied to micro-integrated modules with an MOPA pump source and different crystals for SHG. At an emission wavelength of 560 nm, rectangular millisecond pulses with a peak power of more than 245 mW are achieved.

A volumetric full-color display realized by frequency upconversion of a transparent composite incorporating dispersed nonlinear optical crystals

Popular three-dimensional (3D) TV or film media primarily relies on misleading our visual system by presenting our two eyes with spatially offset two-dimensional (2D) images. In comparison, volumetric displays generate moving objects in three physical dimensions with unlimited viewing angles. In a static volumetric display, voxels instead of pixels are usually addressed by luminescence, scattering or deflection. Although various prototype volumetric display technologies have been developed, the generation of full-color moving objects remains a challenge. Herein, we demonstrate the generation of voxels by frequency upconversion based on second-harmonic generation (SHG) in nonlinear optical crystals that are dispersed in solid-state composite materials that serve as a transparent solid display. Notably, voxels that radiate all colors with near-monochromatic color purity can be created by pumping at different near-infrared wavelengths and thus enable a simple solution to realize a full-color display. A computer-controlled scanner allows the generation of moving 3D objects that are viewable from any direction in a prototype device at a 25 × 25 × 25 mm3 scale, and larger displays that are based on the colloidal dispersion of SHG crystals are envisioned. Our methodology may have important implications for the application of the transparent crystal-in-glass composites in both 3D and 2D display technologies.

A strategic approach to physico-chemical analysis of bis (thiourea) lead chloride – A reliable semi-organic nonlinear optical crystal

Good quality crystals of bis thiourea lead chloride (BTLC) have been grown by slow evaporation method from aqueous solution. Orthorhombic structure and Pna21 space group of the crystals have been identified by single crystal X-ray diffraction. Studies on nucleation kinetics of grown BTLC has been carried out from which meta-stable zone width, induction period, free energy change, critical radius, critical number and growth rate have been calculated. The experimental values of interfacial surface energy for the crystal growth process have been compared with theoretical models. Ultra violet transmittance studies resulted in a high transmittance and wide band gap energy suggested the required optical transparency of the crystal. The second harmonic generation (SHG) and phase matching nature of the crystal have been justified by Kurtz-Perry method. The SHG nature of the crystal has been further attested by the higher values of theoretical hyper polarizability. The dielectric nature of the crystals at different temperatures with varying frequencies has been thoroughly studied. The activation energy values of the electrical process have been calculated from ac conductivity study. Solid state parameters including valence electron plasma energy, Penn gap, Fermi energy and polarisability have been unveiled by theoretical approach and correlated with the crystal's SHG efficiency. The values of hardness number, elastic stiffness constant, Meyer's Index, minimum level of indentation load, load dependent constant, fracture toughness, brittleness index and corrected hardness obtained from Vicker's hardness test clearly showed that the BTLC crystal has good mechanical stability required for NLO device fabrication.

Transverse single-shot cross-correlation scheme for laser pulse temporal measurement via planar second harmonic generation.

We present a novel single-shot cross-correlation technique based on the analysis of the transversally emitted second harmonic generation in crystals with random distribution and size of anti-parallel nonlinear domains. We implement it to the measurement of ultrashort laser pulses with unknown temporal duration and shape. We optimize the error of the pulse measurement by controlling the incident angle and beam width. As novelty and unlike the other well-known cross correlation schemes, this technique can be implemented for the temporal characterization of pulses over a very wide dynamic range (30 fs-1ps) and wavelengths (800-2200 nm), using the same crystal and without critical angular or temperature alignment.

Thermo-physical properties of nonlinear optical crystal K3B6O10Br

The thermo-physical properties of the nonlinear optical (NLO) crystal K3B6O10Br (KBB) were experimentally investigated, including specific heat, thermal conductivity, coefficient of thermal expansion and refractive index. The specific heat of KBB is lower than that of LiB3O5 and higher than that of other borate NLO crystals, such as β-BaB2O4, CsLiB6O10 and CsB3O5, and KBB manifests a high damage threshold because of its lower temperature gradient during laser pulse irradiation. The thermal expansion coefficients were obtained as α x = 5.09 × 10−6 K−1 and α z = 2.39 × 10−5 K−1, showing weaker anisotropy than those of commonly used NLO crystals. The temperature-dependent Sellmeier dispersion equations of the refractive indices were also obtained, and the phase-matching angles for second harmonic generation (SHG) at temperatures of 313, 343, 373, 403 and 433 K which were calculated from these equations are in good agreement with the experimental values. All results are indicative of the KBB crystal as a novel promising NLO crystal for high power SHG.

Optimizations of the Thickness and the Operating Temperature of LiB$_3$O$_5$, BaB$_2$O$_4$, and KTiOPO$_4$ Crystals for Second Harmonic Generation

Optimizations of the Thickness and the Operating Temperature of LiB$_3$O$_5$, BaB$_2$O$_4$, and KTiOPO$_4$ Crystals for Second Harmonic Generation